Chemokines, a family of small pro-inflammatory cytokines, and their receptors, regulate a variety of immune responses to infection, inflammation and tissue repair. Chemokines are divided between two major families on the basis of relative position of cysteine residues in the mature protein (C-C and C-X-C). Primarily, they are responsible for the directional migration, or chemotaxis, of lymphocytes to specific lymphoid tissues, and the recruitment of leukocytes to the sites of infection or tissue damage. In addition to their chemotactic function, chemokines are implicated in other biological events including embryogenesis, lymphopoiesis, vascularization, and HIV pathogenesis. More recently, it has been established that cancer cells exploit signaling through chemokine receptors for several key steps involved in initiation and progression of primary and metastatic cancer. Different types of cancers express different CC and CXC chemokine receptors. There is one chemokine receptor, however, that appears to be expressed by the majority of cancer types, namely, CXCR4.
The CXCR4/CXCL12 Axis
The chemokine receptor CXCR4 is a G-protein coupled receptor that is expressed in a wide assortment of normal tissues, and plays a fundamental role in fetal development, mobilization of hematopoietic stem cells and trafficking of naive lymphocytes (Rossi and Zlotnik, 2000). Besides normal tissues, CXCR4 appears to be expressed by at least 23 different epithelial, mesenchymal and hematopoietic cancers, including prostate cancer, and acute and chronic myeloid leukemias (Balkwill, 2004). The chemokine CXCL12 (also known as stromal-derived factor-1, or SDF-1) is CXCR4's only natural ligand. CXCL12 is expressed constitutively in a variety of tissues, including lung, liver, bone marrow and lymph nodes. These organs with highest expression of CXCL12 correlate with common metastatic destinations in many cancers. The chemokine receptor, CXCR4, and its ligand, CXCL12, appear to be an important chemokine axis regulating tumor growth and metastasis (Nagasawa et al., 1994; Muller et al., 2001; Phillips et al., 2003).
Binding of CXCL12 to CXCR4 activates a variety of intracellular signal transduction pathways and effector molecules that regulate cell chemotaxis, adhesion, survival, and proliferation. There are a number of key molecules that mediate signaling through CXCR4, and some of them will be described below.
CXCL12 and CXCR4 stimulate the phosphatidyl-inositol-3-kinase pathway that subsequently activates the protein kinase, Akt. Activated Akt phosphorylates a variety of intracellular targets, functioning to inhibit apoptosis and prolonging survival in different types of cancer cells. Beyond cell survival, Akt has also been implicated in effects of CXCR4 on migration of cells toward CXCL12 and their proliferation.
The mitogen-activated protein (MAP) kinase pathway is another signal transduction pathway regulated by CXCR4. Following stimulation with CXCL12, CXCR4 activates the kinase MEK, which in turn activates ERK1/2 MAP kinases. Activated ERK1/2 kinases phosphorylate transcription factors such as Elk-1; this process increases expression of genes that promote survival and proliferation of cancer cells.
CXCR4 also appears to regulate angiogenesis, the process that is important for both normal physiology and growth of tumors. Mice lacking CXCR4 or CXCL12 have defective formation of blood vessels in the gastrointestinal tract. Pro-angiogenic effect of CXCR4 signaling may be mediated through up-regulation of vascular-endothelial growth factor (VEGF). Thus, another potential function of CXCR4 signaling in tumor development is promotion of blood vessel production.
The CXCR4/CXCL12 Axis in Hematopoietic Stem Cell Mobilization
All mature blood cells are derived from hematopoietic stem cells (HSC) through intermediates that are termed hematopoietic progenitor cells (HPCs). Hematopoietic cells at various stages of differentiation are localized within the bone marrow (BM), their main site of production. Their mobilization between BM and blood is a physiological process, but under steady-state conditions HPCs and HSCs circulate in the blood at frequencies too low to allow for efficient collection of numbers sufficient to transplantation. Recently, the use of peripheral blood as source of HSCs for transplantations has replaced bone marrow as the preferred source of hematopoietic rescue. Stem cell frequencies in blood are considerably increased both in responses to various growth factors and during the recovery phase following myelosuppressive chemotherapy. Increased number of hematopoietic cells in the blood and amelioration of their mobilization ability will improve the efficiency of transplantation and will shorten the time of cytopenia and engraftment.
Granulocyte Colony-stimulating Factor (G-CSF)-mobilized peripheral-blood mononuclear cells are routinely used as a source of hematopoietic stem cells for transplantation. However, this mobilization results in broad inter-individual variations in circulating progenitor cell numbers. Thus, optimal methods to mobilize and collect peripheral-blood progenitor cells for hematopoietic rescue still need to be found.
Over recent years it has become apparent that the interaction between CXCL12 and its receptor, CXCR4, plays pivotal role in mobilization and engraftment of hematopoietic cells (Kollet et al., 2002; Lapidot et al., 2002; Levesque et al., 2003; Peled et al., 1999; Lapidot et al., 2005; Dar et al., 2005). The CXCR4 receptor is widely expressed on many cell types including HSCs and HPCs and the interaction with its ligand seems to be involved in their chemotaxis, homing and survival. The CXCL12/CXCR4 axis was found to be involved in the retention of hematopoietic cells within the bone marrow microenvironment (Kim et al., 1998) and consequently, it was suggested that antagonizing the interactions of marrow-produced CXCL12 with CXCR4 expressed on HSCs might be an effective HSC mobilizing strategy.
CXCR4 Modulators and T-140 Analogs
Various uses of chemokine receptor modulators, including CXCR4 agonists and antagonists, have been described in the art (Princen et al., 2005; Tamamura et al., 2005). For example, the bicyclam drug termed AMD3100, originally discovered as an anti-HIV compound, specifically interacts with CXCR4 in an antagonistic manner. Blocking CXCR4 receptor with AMD3100 results in the mobilization of hematopoietic progenitor cells; when combining AMD3100 with G-CSF, additive effects were detected (Flomenberg et al., 2005; Broxmeyer et al., 2005). AMD3100 is currently undergoing clinical trials to evaluate its ability to increase stem cells available for transplant (Lack et al., 2005). U.S. Pat. No. 6,365,583 discloses a method to treat a subject who would be benefited by elevation of white blood cell count which method comprises administering to said subject a cyclic polyamine such as AMD3100. Martin et al. (2003) show that the mobilization of neutrophils from the bone marrow by the CXCR2-chemokine, KC, was enhanced by AMD3100, examined 60 minutes after administration to normal BALB/c mice.
U.S. Patent Application Publication No. 2004/0209921 discloses heterocyclic compounds that bind to chemokine receptors, including CXCR4 and CCR5, which may possess protective effects against infection of target cells by a human immunodeficiency virus (HIV). Other potential uses for these compounds suggested by '921 are enhancing the population of progenitor and/or stem cells, stimulating the production of white blood cells, and/or effecting regeneration of cardiac tissue.
U.S. Pat. No. 6,946,445 discloses CXCR4 antagonists comprising the sequence KGVSLSYR. The antagonists disclosed by the '445 patent are suggested to be potentially useful for reducing interferon gamma production by T-cells, treatment of an autoimmune disease, treatment of multiple sclerosis, treatment of other neurological diseases, treatment of cancer, and regulation of angiogenesis. U.S. Pat. No. 6,875,738 discloses methods for treating a solid tumor in a mammal and for inhibiting angiogenesis in a mammal using these antagonists.
U.S. Patent Application Publication No. 2005/0002939 discloses a method of treating ovarian cancer in a mammal, the method comprising administering to the mammal a therapeutically effective dose of a CXCR4 inhibitor. The '939 application suggests that an anti-CXCR4 antibody may impact the survival or growth of a CXCR4-expressing tumor derived from a bladder tumor cell line in a mouse model.
T-140 is a 14-residue synthetic peptide developed as a specific CXCR4 antagonist that suppresses HIV-1 (X4-HIV-1) entry to T cells through specific binding to CXCR4 (Tamamura et al., 1998). Subsequently, peptide analogs of T-140 were developed as specific CXCR4 antagonist peptides with inhibitory activity at nanomolar levels (see Tamamura et al., 2003, WO 2002/020561 and WO 2004/020462).
WO 2002/020561 discloses novel peptide analogs and derivatives of T-140. The '561 publication demonstrates that the claimed peptides are potent CXCR4 inhibitors, manifesting high anti-HIV virus activity and low cytotoxicity.
WO 2004/020462 discloses additional novel peptide analogs and derivatives of T-140, including 4F-benzoyl-TN14003 (SEQ ID NO:1). The '462 publication further discloses novel preventive and therapeutic compositions and methods of using same utilizing T-140 analogs for the treatment of cancer and chronic rheumatoid arthritis. The specification of '462 demonstrates the ability of these peptides to inhibit cancer cell migration, including breast cancer and leukemia cells, and to inhibit metastasis formation in vivo. Further demonstrated therein is inhibition of delayed-type hypersensitivity reaction in mice and collagen-induced arthritis, an animal model of rheumatoid arthritis.
WO 2004/087068 is directed to a method for treating or preventing a CXCR4 mediated pathology comprising administering a CXCR4 peptide antagonist to a host in an amount sufficient to inhibit CXCR4 signal transduction in a cell expressing a CXCR4 receptor or homologue thereof, wherein the CXCR4 peptide antagonist is not an antibody or fragment thereof. The '068 publication discloses that exemplary CXCR4 peptide antagonists include T140 and derivatives of T140, and that the pathology includes cancer such as breast, brain, pancreatic, ovarian, prostate, kidney, and non-small lunch cancer. Other publications directed to the use of CXCR4 antagonists in cancer therapy include, for example, WO 00/09152, US 2002/0156034, and WO 2004/024178.
A recent publication by some of the inventors of the present invention (Avniel et al., 2006) discloses that blocking the CXCR4/CXCL12 axis by a T-140 analog resulted in a significant reduction in eosinophil accumulation in the dermis and improved epithelialization, thus significantly improving skin recovery after burns.
None of the prior art discloses or suggests that CXCR4 inhibitor peptides belonging to the T-140 analog family may also affect CXCR4 activity in an agonist manner. There exists a long felt need for compositions and methods useful for modulating CXCR4-mediated processes involved in pathological conditions in vivo.